The present invention relates to a digital color signal modulating apparatus that modulates a digital color signal to convert digital color image data to an analogue signal for a common color television set.
Color image processing circuit operating in digital computers have employed a R (red) signal, a G (green) signal, and a B (blue) signal, which are combined to express a color image. On the contrary, the color television set has employed a luminance signal, and a chrominance signal, which are combined to be transmitted with a radio wave, and more specifically employs the luminance signal (Y signal), and difference signals to represent the differences between each component of the chrominance signal and the luminance signal. For transmission to the color television set, the signals processed in the computer require a change in signal form. In this case the following signal components are in advance: calculated a U signal component, which denotes the B signal component minus the Y signal component, and a V signal component, which denotes the R signal component minus the Y signal component. The U signal component is modulated with a sine wave, while the V signal component is modulated with a cosine wave. The two signal components are transmitted to the color television set after addition thereof.
Incidentally, the conventional digital color signal modulating apparatus described above has a problem as follows. To modulate the U signal component in the digital color signal with the sine wave, as well as to modulate the V signal component with the cosine wave, there requires a circuit for receiving the U and V signal components, and for multiplying these signal components and those waves, respectively. This necessitates a ROM (Read Only Memory) to store a sine value and another ROM to store a cosine value to execute the trigonometric function.
Furthermore, the multiplication depends upon the clock signal in the computer. In order to perform the multiplication at the same frequency as the predetermined frequency of the color sub-carrier wave, there must be various ROMs suitable for various clock frequencies in various computers; namely, each of the ROMs must provide the specific values of the trigonometric function independent of the respective clock frequency. Therefore, some sorts of the ROMs are essential for applications to various computers having different clock frequencies.
Consequently, a ROM used in a digital color signal modulating apparatus differs from another ROM involved in another digital color signal modulating apparatus, and further those ROMs differs from still another ROM in still another digital color signal modulating apparatus; in short, the types of digital modulating apparatus increase with the types of ROMs corresponding to the various clock frequencies.
On the contrary, a digital color signal modulating apparatus capable of working at various clock frequencies requires the various ROMs, thereby being prone to enlargement.